config: use Rails 5.1 defaults

New or changed defaults are:

- config.assets.unknown_asset_fallback: false
- config.action_view.form_with_generates_remote_forms: true

config/application.rb

@@ -11,7 +11,8 @@ Dotenv::Railtie.load
 
 module TPS
   class Application < Rails::Application
-    config.load_defaults 5.0
+    config.load_defaults 5.1
+
     # Settings in config/environments/* take precedence over those specified here.
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.